The Los Angeles Times reports that the weird, gooey beasts, called salp, usually stay far off the coasts, but for some unknown reason they've migrated to the Diablo Canyon plant in San Luis Obispo. The plants operator, PG&E has been forced to cut production because of the event:

Workers on Monday discovered an influx of the creatures, called salp, clogging screens that are used to keep marine life out of the seawater used as a coolant, Cuddy said. Often thronging many square miles of ocean in huge, gelatinous masses, salp are tubular, transparent organisms that can be roughly the size of a human thumb. No one knows how many are at the Avila Beach plant or how long they will remain.

The creatures aren't each that big—there are just too many of them. According to officials, the salp will likely disperse after a few days. Apparently it's not that weird for jellyfish to swarm power plants. It's just that no one seems to have a good explanation.